Check module signatures with the platform keyring (if enabled)
    
    Upstream has made a keyring to the platform keys. The "KEYS: Allow
    unrestricted boot-time addition of keys to secondary keyring" is
    available upstream for the platform keyring.
    
    The only issue is that module signatures aren't checked with the
    platform keyring, so this introduces a patch to add that which has been
    sent upstream. At least our carried-patch count hasn't gone up.